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Massive black holes common in early Universe

2 June 2004

By Stephen Battersby

Giant black holes were common in the early Universe, according to new observations, but most of them are buried in dust.

Teams from the US and Europe have found hundreds of these hidden giants by combining data from several telescopes, including some ground-based instruments and the big three in space&colon; Hubble, the Chandra X-ray telescope, and the new Spitzer infrared telescope. These instruments cover a wide range of wavelengths and so together they can discern the signature of a hidden quasar.

Quasars are powered by supermassive black holes, around a billion times the mass of the Sun, usually in the core of very distant galaxies. Gas gets sucked in and heats up as it spirals towards the hole, shining brightly.

But most of that radiation is blocked by a fat torus of dust and gas thought to be wrapped around the black hole, like a ring doughnut a few light years wide. Unless our line of sight happens to run through the hole in the doughnut, the central quasar is hidden.

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Some X-rays should slip through the torus, however, as should some long-wave infrared. So the trick is to compare X-ray, visible and infrared images of the sky. The hidden quasars show up as sources that are strong in infrared and X-rays, but weak in visible wavelengths.

Virtual observatory

A few hidden quasars have been detected before, but this is the first time a real survey of their numbers has been possible. The group led by Mark Dickinson of the National Optical Astronomy Observatory, in Tucson, Arizona, saw about 200 quasars altogether.

Another 30 were found by a team led by Paolo Padovani of the European Southern Observatory in Garching, Germany. His team made their discoveries using the Astrophysical Virtual Observatory (AVO), a new piece of software that conveniently combines archived observations from real telescopes. They are publishing the first refereed paper based entirely on virtual observatory data, says Padovani.

About three quarters of the quasars in the US study are hidden, says team member Anton Koekemoer of the Space Telescope Science Institute in Baltimore, Maryland. That confirms the torus idea.

But it also throws up a puzzle. Some of these black holes exist only about 1.5 billion years after the Big Bang, and it is hard for theorists to explain how such massive black holes could grow so quickly.

Bright and short-lived

A small clue comes from the US study. In 2003, Koekemoer’s group reported seven mysterious X-ray sources that are invisible in Hubble images. Now the infrared images from Spitzer have shown that they are entire galaxies surrounded by dust, which blocks almost all their light.

These dust clouds are several thousand light years across – an entirely different scale from the toruses around the black holes.

Koekemoer thinks the dust could be indirectly linked to the growth of the black hole. “These galaxies probably had very active star formation very recently, which generates a lot of dust. Maybe that star formation was also involved in black hole formation, helping them grow so fast.”

Rapid star formation would mean a lot of bright, short-lived stars ending their lives as black holes, which could merge into the giant black hole at the galaxy’s centre.